I have a task of solving a basic channel flow problem using RAS Model (kEpsilon model).
First I started with channelFoam which is an LES solver, I edited the solver code file channelFoam.C, createFields.H, Make/files and Make/options.

In channelFoam.C,

I did the following changes,
1. I commented out #Include "LESModel.H" and added #Include "RASModel.H"
2. I added #Include "turbulenceModel.H"
3. Changed the parameter inside the while loop from runtime.loop() to runtime.run().
4. Added #Include "setDeltaT.H" and runtime++ inside the loop.
5. After that in line 79, I changed divDevBeff(U) to divDevReff()

In createFields.H,
I replaced LESModel with RASModel in lines 42 and 40.

Similarly I did all the changes required to be done in Make/files and Make/options. I compiled the solver and it worked.